1. Field of the Invention
The present invention relates to a high-strength hot-rolled steel sheet superior in stretch-flanging performance and fatigue resistance and a method for production thereof. Owing to its good workability and fatigue resistance, this hot-rolled steel sheet finds use as a raw material for automotive parts such as chassis and suspension systems (including arms and members).
2. Description of the Related Art
The high-strength steel sheet used as a raw material of automotive parts usually has a metallographic structure of dual phase. A dual phase steel sheet, which is composed of a ferrite phase and a martensite phase dispersed therein, is renowned for its good fatigue resistance. There has recently been proposed a way of improving fatigue resistance by introduction of retained austenite into the metallographic structure. Unfortunately, the dual phase steel sheet and retained austenite steel sheet are good in fatigue resistance but poor in stretch-flanging performance and hence are difficult to work.
Any steel sheet used for automotive suspension parts is required to have high strength and good fatigue resistance after it has been made into finished products. Moreover, it needs good workability to facilitate complex forming. Particularly, it needs good stretch-flanging performance (hole expanding performance). However, the above-mentioned dual phase steel sheet and retained austenite steel sheet do not meet these requirements. In other words, there has been no steel sheet which has high strength and meets requirements for both stretch-flanging performance and fatigue properties.
With the foregoing in mind, the present inventors have been investigating the improvement of hot-rolled steel sheet in strength and stretch-flanging performance. They proposed a method for improvement in Japanese Patent Laid-open Nos. 172924/1994, 11382/19995, and 70696/1995 based on the results of their investigation on the chemical composition and metallographic structure of low-carbon steels.
Although their investigation achieved improvement in strength and stretch-flanging performance to some extent, it is still difficult to improve both of them simultaneously because they are contradictory to each other. In addition, a steel product to be used for automotive parts (as in the present invention) needs good workability (as typified by stretch-flanging performance) as well as good fatigue resistance for safety. There is plenty of room for further improvement, particularly in stretch-flanging performance.
The present invention was completed in view of the above-mentioned situation. It is an object of the present invention to provide a hot-rolled steel sheet having high strength as well as good workability, particularly good stretch-flanging performance. It is another object of the present invention to provide a hot-rolled steel sheet having good fatigue resistance.